Chillers equipped with variable speed and variable geometry compressors, such as centrifugal compressors with variable frequency drives and variable inlet guide vanes, present unique opportunities and challenges with respect to control of chiller capacity and efficiency optimization. For any given chiller capacity there are an infinite number of pairs of compressor speed and geometry that could provide a desired chiller capacity. There are also an infinite number of compressor speed and vane position pairs that result in undesirable compressor surge. In general, compressor efficiency can be maximized by operating the compressor at or near its surge boundary. The inventor has discovered that control techniques which follow conventional wisdom with respect to compressor efficiency presents unexpected controls challenges and may create unexpected controls aberrations. For example, over certain capacity ranges, conventional control techniques may attempt to reduce chiller capacity, but instead increase it and vice versa. Control limit cycles may also be encountered. The capacity control techniques disclosed herein provide unexpected benefits with respect to controls and efficiency. The demand for ever increasing levels of efficiency and reliability creates a significant need for the unique and inventive chiller capacity control apparatuses, systems and methods disclosed herein.